The present invention relates to surface-active compositions for dispersing oil slicks. This invention is also directed to a method for treating and dispersing oil slicks.
Pollution of sea water by oil (crude oil or fractions of crude oil) occasioned by accidents, offshore drilling operations, discharge of ballast water or spillage from oil tankers, results in the formation of a continuous film or slick of oil which tends to continuously spread. This oily film is undesirable because it constitutes a barrier to the transfer of air and light from the atmosphere which are indispensable to support marine life.
One way of dealing with such pollution problems is to use oil collecting compositions. They are poured at the periphery of the oil slick, and they repel the oil which is gathered into a narrower area and may be removed by usual mechanical means. However, this oil-spill compression technique is only suitable on a relatively calm sea. Moreover, the compositions are only efficient during a very short period of time, due to the solubility of the compositions in the oil.
Another technique involves the use of surface-active agents which disintegrate the cohesive oily film into small droplets and disperse the droplets into the water column to a depth of several meters under the sea surface. The film is thus broken, and there is again a transfer of air and light from the atmosphere.
Some surface-active agents are highly effective as oil-spill dispersants, but their use has resulted in severe damage to marine life because they are too toxic towards aquatic flora and fauna. Other surface-active agents having a lower degree of toxicity are very often ineffective as oil dispersants in open seas. In fact, they are too quickly washed out by the water from the oil slick on which they are sprayed with the result that the droplets of oil coalesce or re-agglomerate and again form a slick.
For this reason, surface-active agents are generally used in admixture with a solvent, particularly a solvent having a low degree of toxicity, such as paraffinic and cycloparaffinic hydrocarbons and lower alcohols. The use of a solvent presents many advantages: it decreases the pour point of the composition, it reduces the viscosity of the oil forming the slick, it acts as a diluent for the surface-active agent which is more readily and uniformly distributed into the oil slick, and it reduces the time required to mix the dispersant agent and the oil.
Another problem which arises from the use of surface-active agents as oil spill dispersants is their water-solubility. Some of these agents dissolve too quickly in water and thus do not penetrate the oil slick. For this reason, it has been suggested to use dispersant compositions containing at least two surface-active agents. Better dispersion results from the use of blends of surface-active agents whose proportions are selected so that the hydrophilic-lipophilic balance (or HLB) falls between defined limits.
The trend therefore is to use blends of surface-active agents in admixture with a solvent. British Pat. No. 1,404,684 describes such a composition in which the oil slick dispersant comprises a blend of fatty acid polyoxyalkylene glycol esters and fatty acid sorbitan esters (HLB of the blend: from 9 to 10.5), dissolved in a hydrocarbon solvent containing less than 3 weight percent of aromatics. The solution may contain from 5 to 50 weight percent of the surfactant blend, a proportion of 8 to 10 weight percent being particularly preferred. Other patents describe the use of compositions comprising a fatty acid sorbitan monoester and a polyalkylene oxide adduct of fatty acid sorbitan monoester (British Pat. No. 1,255,394) or comprising these compounds in admixture with 25 to 35 volume percent (exclusive of solvent) of a 75% aqueous solution of dialkyl sulfosuccinate salt (U.S. Pat. No. 3,793,218). The best results in dispersing oil slicks are obtained in formulations containing about 40 percent of a blend of surface-active agents and about 60 percent of an isoparaffinic hydrocarbon solvent.
The use of a solvent presents the aforementioned advantages, but some problems may occur when dispersant blends are diluted by large amounts of solvent. Storage of compositions containing volatile solvents can increase the hazard of fire and lead to explosions. The presence of a large amount of solvent in the composition means that a lower amount of surface-active agent is available for dispersing the oil slick. Moreover, some volatile components of the solvent may be lost by evaporation when the composition is applied by spraying on the oil slick, particularly when the composition is applied from an aircraft or from a distant boat, and the advantages resulting from the incorporation of a solvent in the composition are partially lost.
In order to be effective in dispersing oil slicks, a surface-active composition in the form of a solution of surface-active agents should fulfill the following conditions:
have a high content in active agents, PA1 have a sufficient and long-lasting dispersing action, PA1 be a homogeneous and limpid liquid, free from insoluble material and from clouding, PA1 be handleable at low temperatures and have an adequate viscosity, allowing the composition to be sprayed from boats or from aircraft without any increase in viscosity due to evaporation of volatile solvent during its application, PA1 have a flash point higher than 61.degree. C., to meet the regulations concerning the handling of chemical products, PA1 be non-toxic and biodegradable, PA1 be easily manufactured and at low cost.